They fill it in and give you their email address.
You then email them a link to a ‘hidden’ page that contains a link to download the file.
Name the page something like https://www.mysite.com/lksfjhgld so nobody is likely to stumble across it and make sure you un-check ‘Show in navigation’ in General settings.

Of course, this requires manual intervention by you and isn’t the best way to secure a high value file. But if it’s something like a free how-to PDF booklet, then it would work.

There are many more robust ways to secure pages but this would be simple to implement and free.

Thinking about it - if there was a way for the submit button on the form to link to the page for the download that would pretty much nail what I need. Any idea if there’s a work around to achieve that?